Code Division Multiple Access (CDMA) networks are widely deployed throughout the world. The current implementations of CDMA typically follow the IS-95 industry standards and are referred to as IS-95 wireless systems. With the advent of enhancements to CDMA technology such as third generation CDMA, CDMA2000 and W-CDMA, the deployment of CDMA is expected to increase dramatically.
A typical CDMA system 100 is shown in FIG. 1. It is divided into a plurality of cells 121. Each cell contains a fixed base station 103. Each base station 103 is connected to a centralized switch or mobile switching center 109 that provides switching capabilities and acts as a gateway to wired networks such as the public switched telephone network (PSTN), the Internet, and other public and private data communications networks. As is known, the base station 103 includes a transmitter 105 and a receiver 107 for communicating with the mobile customers or users.
On the customer side, users connect to the wireless network through wireless mobile nodes 101 that can act as transmitters and receivers. The mobile nodes 101 communicate with the base stations 103 over wireless communications links. The link from a base station transmitter 105 to a mobile node receiver is the forward link 115 (or downlink). The link from the mobile node transmitter to a base station receiver 107 is referred to as the reverse link 113 (or uplink).
One advantage of CDMA over other wireless access systems is that all users share the same spectrum at the same time. However, the fact that multiple users occupy the same bandwidth limits performance and capacity. Because the conventional matched filter receiver 107 does an imperfect job of removing signals from these users, each user in a CDMA system degrades the performance of every other user; this effect is called multiple access interference or MAI. An increase in interference between users can lower the ability of a wireless provider to reuse frequencies, resulting in a reduction of system capacity. Because of the tremendous demand for wireless voice and data services and increased competition between service providers, CDMA network providers cannot afford such a reduction in system capacity. Therefore, wireless providers are continually striving to maximize system capacity, which in turn, requires limiting interference.
In CDMA wireless systems, power control is used to control the level of MAI at the base station. By adjusting every user's power so that all user transmissions arrive at the base station at approximately the same level, the base station receiver for each user sees the same amount of MAI, and the link quality is roughly the same for each user. If power control was not implemented, then a single user close to the base station could prevent the conventional CDMA receiver for other users from receiving a usable signal, resulting in the so-called near-far problem.
Power control works reasonably well for currently deployed CDMA wireless systems although limitations in the speed of power control are a constant engineering concern and limit capacity and link quality. However, there are frequently situations where it is desirable to deploy auxiliary receivers that are not the target of mobile station power control. Auxiliary receivers can be used to monitor the health of a CDMA wireless system or assist in geolocation. These auxiliary receivers may even be used by law enforcement and military operators for non-cooperative monitoring of a CDMA system for drug-interdiction, counter-terrorism and international intelligence gathering. In these cases, the auxiliary receiver must contend with a wide range of received power levels. Often the auxiliary receiver may need to receive a signal from a mobile station whose received power level is far below (30 dB or more) below the strongest arriving signal.
A need therefore exists for enabling a user in a CDMA system to receive user signals in the presence of interference from other users when the power level of all co-channel signals is not adjusted to be substantially the same.